Organic light emitting display (OLED)

ABSTRACT

An Organic Light Emitting Display (OLED) that is capable of decreasing the number of output lines for a data driver using a demultiplexer, displaying an image with uniform brightness, and adjusting white balance includes: a plurality of red, green and blue sub-pixels, and each sub-pixel is provided with an auxiliary capacitor to compensate a decreased driving voltage. The respective auxiliary capacitors are different in capacitance according to emission efficiency of the red, green and blue sub-pixels. The auxiliary capacitor of the green sub-pixel has a larger capacitance than the auxiliary capacitor of the red sub-pixel, and the auxiliary capacitor of the red sub-pixel has a larger capacitance than the auxiliary capacitor of the blue sub-pixel. This enables the white balance to be adjusted with the same data voltage.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C. §119 from an applicationfor ORGANIC ELECTROLUMINESCENT DISPLAY DEVICE earlier filed in theKorean Intellectual Property Office on the 16 of Aug. 2005 and thereduly assigned Serial No. 10-2005-0074865.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Organic Light Emitting Display(OLED), and more particularly, to an OLED that is capable of decreasingthe number of output lines for a data driver using a demultiplexer,displaying an image with uniform brightness, and adjusting whitebalance.

2. Description of the Related Art

Recently, various flat panel displays have been developed asalternatives to a relatively heavy and bulky cathode ray tube. FlatPanel Displays (FPDs) include Liquid Crystal Displays (LCDs), FieldEmission Displays (FEDs), Plasma Display Panels (PDPs), Organic LightEmitting Displays (OLEDs), etc.

Among the FPDs, the OLED includes an organic light emitting diode thatemits light for itself by recombination of electrons from a cathode andholes from an anode. Such an OLED has advantages in that its responsetime is relatively fast (about 1 μs) and its power consumption isrelatively low. Generally, the OLED employs a Thin Film Transistor (TFT)provided in each pixel for supplying a driving current corresponding toa data signal to the organic light emitting diode, thereby allowing theorganic light emitting diode to emit light and display a predeterminedimage.

An OLED includes a display panel, a scan driver, a data driver, and atiming controller.

The display panel includes a plurality of pixels formed in regions wherea plurality of scan lines, a plurality of emission control lines, and aplurality of data lines intersect each other. The respective pixelsreceive a first power supply and a second power supply from the outside,and emit light corresponding to data signals transmitted from theplurality of data lines, thereby displaying a predetermined image.Furthermore, in the pixels, their emission times are controlled byemission control signals transmitted through the emission control lines.

The scan driver generates scan signals in response to a scan controlsignal from the timing controller, and sequentially supplies the scansignals to the plurality of scan lines, thereby selecting the pixels.Furthermore, the scan driver generates emission control signals inresponse to the scan control signal, and sequentially supplies theemission control signals to the plurality of emission control lines,thereby controlling the light emission.

The data driver receives red (R), green (G) and blue (B) data from thetiming controller, generates the data signals in response to a datacontrol signal, and supplies the data signals to the plurality of datalines The data driver supplies the data signal corresponding to onehorizontal line to the data lines per one horizontal period.

The timing controller generates the data control signal and the scancontrol signal in correspondence to video data and horizontal/verticalsynchronous signals supplied from an external graphic controller. Thetiming controller respectively supplies the data control signals and thescan control signals to the data driver and the scan drive.

In an OLED with this configuration, the respective pixels are placed inregions where the plurality of scan lines, the plurality of emissioncontrol lines, and the plurality of data lines intersected each other.The data driver is provided with m output lines to respectively supplythe data signals to m data lines. That is, in this OLED, the data drivermust be provided with the same number of output lines as the number ofdata lines. Accordingly, the data driver internally includes a pluralityof data Integrated Circuits (IC) to have m output lines, therebyincreasing manufacturing cost. In particular, as the resolution and sizeof the display panel increases, the data driver needs more data ICs,thereby further increasing the manufacturing cost.

SUMMARY OF THE INVENTION

The present invention, therefore, provides an Organic Light EmittingDisplay (OLED) that is capable of decreasing the number of output linesfor a data driver using a demultiplexer, displaying an image withuniform brightness, and adjusting white balance with uniform voltage.

In an exemplary embodiment of the present invention, an Organic LightEmitting Display (OLED) having a plurality of pixels each having a redsub-pixel, a green sub-pixel, and a blue sub-pixel is provided, eachsub-pixel including: a pixel driver connected to a data line, a scanline and a first power supply voltage line, and including a storagecapacitor adapted to store a driving voltage supplied via the data line,and to generate a predetermined driving current; and an organic lightemitting diode connected between the pixel driver and a second powersupply voltage line, and adapted to emit light with a brightnesscorresponding to the driving current; and an auxiliary capacitorconnected between the storage capacitor and the scan line, and adaptedto generate a compensation voltage to increase the driving voltage; theauxiliary capacitors of the sub-pixels have different capacitancesaccording to an emission efficiency ratio of their respectivesub-pixels.

The capacitance of the auxiliary capacitor is inversely proportional toa driving current ratio of the sub-pixels to generate a white pixel. Theauxiliary capacitor of the green sub-pixel has a larger capacitance thanthat of the auxiliary capacitor of the red sub-pixel, and the auxiliarycapacitor of the red sub-pixel has a larger capacitance than that of theauxiliary capacitor of the blue sub-pixel.

A compensation voltage increased by the auxiliary capacitor is definedby the following equation:Vx=Caux/(Cst+Caux)*(VVDD-VVSS)where Vx is the compensation voltage, Caux is the capacitance of theauxiliary capacitor, Cst is the capacitance of the storage capacitor,VVDD is a high level scan signal, and VVSS is a low level scan signal.

The pixel driver further includes: an initialization transistorconnected between a first terminal of the storage capacitor and aninitialization voltage line, and adapted to be turned-on by an(n−1)^(th) scan signal to initialize the storage capacitor; a firstswitching transistor connected to the data line, and adapted to beturned-on by an n^(th) scan signal to transmit the data voltage; adriving transistor having a first electrode connected to the firstswitching transistor and a gate electrode connected to a first terminalof the storage capacitor, and adapted to generate the driving current; athreshold voltage compensation transistor connected between the gateelectrode and a second electrode of the driving transistor, and adaptedto be turned-on by the n^(th) scan signal to cause the drivingtransistor be diode-connected and to compensate a threshold voltage ofthe driving transistor; and a second switching transistor connectedbetween the first power supply voltage line and the second electrode ofthe driving transistor, and adapted to be turned-on by an n^(th)emission control signal to transmit the first power supply voltage tothe second electrode of the driving transistor.

The pixel driver further includes an emission control transistorconnected between the driving transistor and the organic light emittingdiode, and adapted to be turned-on by the n^(th) emission control signalto transmit the driving current to the organic light emitting diode.

The first through sixth transistors include N or P MOSFETs having thesame conductivity type.

In another exemplary embodiment of the present invention, an OrganicLight Emitting Display (OLED) is provided including: a display panelhaving a plurality of pixels adapted to display an image; a scan driveradapted to supply a scan signal to the display panel; a data driveradapted to supply a data signal to the display panel; a timingcontroller adapted to generate a data control signal to control the datasignal and a scan control signal to control the scan driver; ademultiplexer coupled between the data driver and the display panel, andadapted to sequentially supply the data signal to correspondingsub-pixels that constitute the pixel; and a demultiplexer controlleradapted to control the demultiplexer.

The demultiplexer is connected to the display panel via data linesconnected to respective sub-pixels. The data lines are respectivelyconnected to auxiliary capacitors and wherein the auxiliary capacitorshave different capacitances according to their respective sub-pixels.

The auxiliary capacitors have different capacitances corresponding to anemission efficiency of their respective sub-pixels. The auxiliarycapacitors of the green, red, and blue sub-pixels have respectivecapacitances in order of green>red>blue.

The demultiplexer sequentially supplies the data signals to eachsub-pixel during a horizontal period.

The scan driver supplies the scan signal to the pixel, such that thedata signals are simultaneously provided to the sub-pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention, and many of theattendant advantages thereof, will be readily apparent as the presentinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings in which like reference symbols indicate the sameor similar components, wherein:

FIG. 1 is a block diagram of an OLED;

FIG. 2 is a block diagram of an OLED according to an embodiment of thepresent invention;

FIG. 3 is an internal circuit diagram of a demultiplexer of FIG. 2;

FIG. 4 is a circuit diagram of a representative pixel among N×M pixelsof FIG. 2;

FIG. 5 is a circuit diagram of a detailed connection structure betweenthe representative demultiplexer of FIG. 3 and the representative pixelsof FIG. 4 according to an embodiment of the present invention;

FIG. 6 is a timing diagram of the pixel circuit of FIG. 5;

FIG. 7 is a simulation graph of changes in a driving current accordingto the capacitance of an auxiliary capacitor in a pixel of FIG. 4; and

FIG. 8 is a layout diagram of red, green and blue sub-pixels capable ofadjusting white balance with the same voltage based on the simulation ofFIG. 7 according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an OLED. Referring to FIG. 1, the OLEDincludes a display panel 10, a scan driver 20, a data driver 30, and atiming controller 40.

The display panel 10 includes a plurality of pixels P11 through Pnmformed in regions where a plurality of scan lines S1 through Sn, aplurality of emission control lines E1 through En, and a plurality ofdata lines D1 through Dm intersect each other. The respective pixels P11through Pnm receive a first power supply Vdd and a second power supplyVss from the outside, and emit light corresponding to data signalstransmitted from the plurality of data lines D1 through Dm, therebydisplaying a predetermined image. Furthermore, in the pixels P11 throughPnm, their emission times are controlled by emission control signalstransmitted through the emission control lines E1 through En.

The scan driver 20 generates scan signals in response to a scan controlsignal Sg from the timing controller 40, and sequentially supplies thescan signals to the plurality of scan lines S1 through Sn, therebyselecting the pixels P11 through Pnm. Furthermore, the scan driver 20generates emission control signals in response to the scan controlsignal Sg, and sequentially supplies the emission control signals to theplurality of emission control lines E1 through En, thereby controllingthe light emission.

The data driver 30 receives red (R), green (G) and blue (B) data fromthe timing controller 40, generates the data signals in response to adata control signal Sd, and supplies the data signals to the pluralityof data lines D1 through Dm. The data driver 30 supplies the data signalcorresponding to one horizontal line to the data lines D1 through Dm perone horizontal period.

The timing controller 40 generates the data control signal Sd and thescan control signal Sg in correspondence with video data andhorizontal/vertical synchronous signals Hsync and Vsync supplied from anexternal graphic controller (not shown). The timing controller 40respectively supplies the data control signals Sd and the scan controlsignals Sg to the data driver 30 and the scan driver 20.

In an OLED with this configuration, the respective pixels P11 throughPnm are placed in regions where the plurality of scan lines S1 throughSn, the plurality of emission control lines En through Em, and theplurality of data lines D1 through Dm intersect each other. The datadriver 30 is provided with m output lines to respectively supply thedata signals to m data lines D1 through Dm. That is, in this OLED, thedata driver 30 must be provided with the same number of output lines asthe number of data lines D1 through Dm. Accordingly, the data driver 30internally includes a plurality of data integrated circuits (IC) to havem output lines, thereby increasing manufacturing cost. In particular, asthe resolution and size of the display panel 10 increases, the datadriver 30 needs more data ICs, thereby further increasing themanufacturing cost.

Hereinafter, exemplary embodiments of the present invention aredescribed with reference to accompanying drawings.

FIG. 2 is a block diagram of an OLED according to an embodiment of thepresent invention. Referring to FIG. 2, an OLED according to anembodiment of the present invention includes a display panel 100, a scandriver 120, a data driver 130, a timing controller 140, a demultiplexerunit 150, and a demultiplexer controller 160.

The display panel 100 includes a plurality of pixels P111 through Pnmkwhich are placed in regions defined by a plurality of scan lines S1through Sn, a plurality of emission control lines E1 through En, and aplurality of data lines D11 through Dmk.

The respective pixels P111 through Pnmk emit light corresponding to datasignals supplied through the corresponding data lines D11 through Dmk.Among the pixels P111 through Pnmk, a representative pixel 110 will bedescribed later.

Furthermore, the data lines D11 through Dmk corresponding to the pixelsP111 through Pnmk are respectively provided with a plurality of dataline capacitors C_(data11) through C_(datamk) to temporarily store thedata signals.

For example, during a data program period, when a data voltage issupplied to the 1^(st) data line D11 to cause the 1^(st) pixel P111 emitlight, the 1^(st) data line capacitor C_(data11) formed in the data lineD11 stores the data voltage temporarily. Next, during a scan period,when the 1^(st) pixel P111 is selected by the 1^(st) scan signal S1, thedata voltage stored in the 1^(st) data line capacitor C_(data11) issupplied to the 1^(st) pixel P111, thereby causing the 1^(st) pixel P111emit the light corresponding to the data voltage.

Thus, the data line capacitors C_(data11), through C_(datamk) formed inthe data lines D11 through Dmk respectively temporarily store the datasignals to be supplied to the plurality of data lines D11 through Dmk.Furthermore, the data line capacitors C_(data11) through C_(datamk)supply the stored data voltage to the pixels P111 through Pnmk selectedby the scan signals. The data line capacitors C_(data11) throughC_(datamk) are achieved by parasitic capacitors equivalently formed bythe data lines D11 through Dmk, a third electrode, and an insulatinglayer interposed therebetween. Substantially, each of the data linecapacitors C_(data11) through C_(datamk) equivalently formed in the datalines D11 through Dmk preferably has a capacitance larger than thecapacitance of a storage capacitor Cst provided in each of the pixelP111 through Pnmk in order to stably store the data signal.

The scan driver 120 generates the scan signals in response to scancontrol signals Sg supplied from the timing controller 140, andsequentially supply the scan signals to the scan lines S1 through Sn.The scan driver 110 supplies the scan signal only in a predeterminedperiod (i.e., the scan period) of one horizontal period 1H as shown inFIG. 6. According to an embodiment of the present invention, onehorizontal period 1H is divided into the scan period and the dataprogram period. The scan driver 120 supplies the scan signal to the scanline Sn in the scan period of one horizontal period, and does not supplythe scan signal in the data program period. Furthermore, the scan signalcan be supplied during the data program period. So, the scan period canbe overlapped with the data program period. The scan driver 120generates the emission control signals in response to the scan controlsignals Sg, and sequentially supplies the emission control signals tothe emission control lines E1 through En.

The data driver 130 receives R, G and B data from the timing controller140, and sequentially supplies R, G and B data signals to output linesD1 through Dm in response to data control signals Sd. The data driver130 sequentially supplies k data signals (where k is an integer largerthan 2, e.g., k is three R, G and B data signals as shown in FIG. 6) tothe output lines D1 through Dm connected to respective output terminals.In more detail, the data driver 130 sequentially supplies the datasignals (e.g., the R, G and B data) to the corresponding pixels in thedata program period of one horizontal period 1H. The data signals (R, Gand B) to be supplied to the corresponding pixels are supplied onlyduring the data program period.

The timing controller 140 generates the data control signals Sd and thescan control signals Sg in correspondence with video data and horizontaland vertical synchronous signals supplied from an external graphiccontroller (not shown). The timing controller 140 respectively suppliesthe data control signals Sd and the scan control signals Sg to the datadriver 130 and the scan driver 120.

The demultiplexer unit 150 includes m demultiplexers 151. Specifically,the demultiplexer unit 150 includes the same number of demultiplexers151 as the number of output lines D1 through Dm connected to the datadriver 130, and input terminals of the demultiplexers 151 are connectedto the output terminals D1 through Dm of the data driver 130,respectively. Furthermore, an output terminal of the demultiplexer 151,e.g., the 1^(st) demultiplexer is connected to k data lines D11 throughD1 k. The 1^(st) demultiplexer 151 applies k data signals, which aresequentially supplied in the data program period, to k data lines D11through D1 k. When the k data signals sequentially supplied to oneoutput line D1 are sequentially supplied to the k data lines D11 throughD1 k, the number of output lines provided in the data driver 130 can beremarkably decreased. For example, when k=3, the number of output linesprovided in the data driver 130 is divided by 3, and thus the number ofdata ICs provided in the data driver 130 is also reduced. According toan embodiment of the present invention, the demultiplexer 151 is usedfor supplying the data signal corresponding to one output line D1 to kdata lines D11 through D1 k, thereby reducing the manufacturing cost ofthe data IC.

The demultiplexer controller 160 supplies k control signals to controlterminals of the demultiplexer 151 in the data program period of onehorizontal period 1H such that k data signals are divided and suppliedthrough the output line D1 to k data lines D11 through D1 k. The kcontrol signals supplied from the demultiplexer controller 160 aresequentially supplied as shown in FIG. 6, without overlapping each otherin the data program period. In this embodiment, the demultiplexercontroller 160 is externally provided in the timing controller 140.Alternatively, the demultiplexer controller 160 can be internallyprovided in the timing controller 140.

FIG. 3 is an internal circuit diagram of the demultiplexer of FIG. 2. InFIG. 3, for convenience of explanation, the demultiplexer is describedon the assumption that k is 3 and data is input in the order of red,green and blue. Furthermore, the demultiplexer is described on theassumption that it is connected to the 1^(st) output line D1 of the datadriver 130.

Referring to FIG. 3, the demultiplexer 151 includes a first switchingelement T1, a second switching element T2, and a third switching elementT3. Each of the switching elements T1, T2 and T3 can be made of a ThinFilm Transistor (TFT). In this embodiment, the switching elements T1, T2and T3 are formed of P-channel Metal Oxide Semiconductor Field EffectTransistors (PMOSFETs), but are not limited thereto. Alternatively, theswitching elements T1, T2 and T3 can be formed of n-channel MOSFETs.

The first switching element T1 is connected between the 1^(st) outputline D1 and the 1^(st) data line D11. The first switching element T1 isturned-on when a first control signal CS1 is supplied from thedemultiplexer controller 160, and supplies a red data signal from the1^(st) output line D1 to the 1^(st) data line D11. The data signalsupplied to the 1^(st) data line D11 is stored in the 1^(st) data linecapacitor C_(data11) in the data program period of FIG. 2.

The second switching element T2 is connected between the 1^(st) outputline D1 and the 2^(nd) data line D12. The second switching element T2 isturned-on when a second control signal CS2 is supplied from thedemultiplexer controller 160, and supplies a green data signal from the1^(st) output line D1 to the 2^(nd) data line D12. The data signalsupplied to the 2^(nd) data line D12 is stored in the 2^(nd) data linecapacitor C_(data12) in the data program period of FIG. 2.

The third switching element T3 is connected between the 1^(st) outputline D1 and the 3^(rd) data line D13. The third switching element T3 isturned-on when a third control signal CS3 is supplied from thedemultiplexer controller 160, and supplies a blue data signal from the1^(st) output line D1 to the 3^(rd) data line D13. The data signalsupplied to the 3^(rd) data line D13 is stored in the 3^(rd) data linecapacitor C_(data13) in the data program period of FIG. 2.

Detailed operations of the demultiplexer 151 are described later inassociation with the structure of the pixel 110.

FIG. 4 is a circuit diagram of a representative pixel among N×M pixelsof FIG. 2. The pixels according to an embodiment of the presentinvention are not limited to the pixel shown in FIG. 4.

Referring to FIG. 4, the pixel 110 according to an embodiment of thepresent invention includes an organic light emitting diode; and a pixeldriving circuit 111 connected to a data line Dmk, previous and currentscan lines Sn−1 and Sn, an emission control line En, a first powersupply voltage line Vdd, and an initialization voltage line Vinit andgenerating driving current to cause the organic light emitting diode toemit light. Furthermore, the data line capacitor C_(datamk) is formed inthe data line Dmk to supply the data voltage to the pixel 110.

The organic light emitting diode has an anode electrode connected to thepixel driving circuit 111, and a cathode electrode connected to a secondpower supply voltage line Vss. A second power supply Vss has a lowervoltage than a first power supply Vdd, e.g., a ground voltage, anegative voltage or the like. Thus, the organic light emitting diodeemits light corresponding to the driving current supplied from the pixeldriving circuit 111.

The pixel driving circuit 111 includes one storage capacitor Cst and sixtransistors M1 through M6. The first transistor M1 acts as a drivingtransistor, the third transistor M3 acts as a threshold voltagecompensation transistor for compensating a threshold voltage by causingthe first transistor M1 be diode-connected, and the fourth transistor M4acts as an initialization transistor for initializing the storagecapacitor Cst. Furthermore, the sixth transistor M6 acts as an emissioncontrol transistor for controlling the emission of the organic lightemitting diode, and the second and fifth transistors M2 and M5 act asswitching transistors.

The first switching transistor M2 has a gate electrode connected to thecurrent scan line Sn and a source electrode connected to the data lineDmk, and is turned-on by the scan signal transmitted through the currentscan line Sn, thereby transmitting the data voltage supplied from thedata line capacitor C_(datamk).

The driving transistor M1 has a source electrode connected to a drainelectrode of the first switching transistor M2, and a gate electrodeconnected to a node N. At the node N, a source or drain electrode of thethreshold voltage compensation transistor M3 and a first terminal of thestorage capacitor Cst are connected in common, and a gate voltage of thedriving transistor M1 is determined. Thus, the driving transistor M1generates a driving current corresponding to the voltage supplied to thegate electrode.

The threshold voltage compensation transistor M3 is connected betweenthe gate and source electrodes of the driving transistor M1, and causesthe driving transistor M1 be diode-connected in response to the scansignal transmitted through the current scan line Sn. Thus, the drivingtransistor M1 acts as a diode according to the scan signal, therebysupplying a voltage of Vdata-Vth [V] to the node N. The voltage ofVdata-Vth [V] is used as the gate voltage of the driving transistor M1.

The initialization transistor M4 is connected between the initializationvoltage line Vinit and the first terminal of the storage capacitor Cst,and discharges electrical charges stored in the storage capacitor Cstduring the previous frame through the initialization voltage line Vinitin response to the scan signal of the (n−1)^(th) scan line Sn−1connected to the gate electrode, thereby initializing the storagecapacitor Cst.

The second switching transistor M5 is connected between the first powersupply voltage line Vdd and the source electrode of the drivingtransistor M1, and is turned-on by the emission control signaltransmitted through the emission control line En connected to the gateelectrode thereof, thereby supplying the first power voltage Vdd to thesource electrode of the driving transistor M1.

The emission control transistor M6 is connected between the drivingtransistor M1 and the organic light emitting diode, and transmits thedriving current to the organic light emitting diode, wherein the drivingcurrent is generated in the driving transistor M1 in response to theemission control signal transmitted through the emission control line Enconnected to the gate electrode thereof.

The storage capacitor Cst is connected between the first power supplyvoltage line Vdd and the gate electrode of the driving transistor M1,and maintains electrical charges corresponding to the voltage ofVdata-Vth[V] supplied between the first power voltage Vdd and the gateelectrode of the driving transistor M1 for one frame.

In FIG. 4, the first through sixth transistors M1 through M6 are formedof PMOSFETs, but are not limited thereto. Alternatively, the firstthrough sixth transistors M1 through M6 can be formed of NMOSFETs.

Thus, a voltage corresponding to the data signal is stored in the dataline capacitor C_(datamk) in the data program period, and the voltagestored in the data line capacitor C_(datamk) is supplied to the pixel inthe scan period, thereby supplying the data signal to the pixel with theforegoing configuration. Because the voltages stored in the data linecapacitors C_(data11) through C_(data1k) are supplied to the pixels atthe same time, respectively, i.e., because the data signals are suppliedat the same time, an image is displayed with uniform brightness.

However, as the data program period and the scan period are separatedusing the demultiplexer, the data line capacitor C_(datamk) and thestorage capacitor Cst of the pixel, which are separated from each otherin the data program period, are connected to each other in the scanperiod, so that the electrical charge corresponding to the data voltageVdata stored in the data line capacitor C_(datamk) is shared between thedata line capacitor C_(datamk) and the storage capacitor Cst. Hence, thegate voltage Vg_(M1), of the driving transistor M1 can be substantiallyobtained by Equation 1.Vg _(M1)=(Cdata*Vdata+Cst*Vinit)/(Cdata+Cst)  Equation 1:where, Vg_(M1), is a gate voltage of the driving transistor M1, Vdata isa data voltage, Vinit is an initialization voltage, Vdd is a first powersupply voltage, Cdata is the capacitance of each data line capacitor,and Cst is the capacitance of each storage capacitor of the pixel.

Referring to Equation 1, the difference between the gate voltageVg_(M1), of the driving transistor M1 and the data voltage Vdata variesdepending on the capacitance of the data line capacitor Cdata and thestorage capacitor Cst. That is, a voltage lower than the data voltagesupplied to the data line is supplied to the gate electrode of thedriving transistor. Thus, it is difficult to display black, so that acontrast ratio is lowered.

This problem can be solved by increasing a black data voltage. However,it is impossible to supply the higher black data voltage because of thespecifications of the data driver. Alternatively, this problem can besolved by lowering the first power supply voltage Vdd. In this case,black can be displayed, but the second power supply voltage Vss must belowered as much as the first power supply voltage Vdd. Therefore, DC/DCefficiency of the second power supply voltage Vss is remarkablydecreased.

Therefore, the pixel according to an embodiment of the present inventionincludes an auxiliary capacitor Caux therein as shown in FIG. 4.

The auxiliary capacitor Caux has a first electrode connected in commonto the current scan line Sn and the gate electrode of the firstswitching transistor M2, and a second electrode connected in common tothe storage capacitor Cst and the gate electrode of the drivingtransistor M1.

The auxiliary capacitor Caux boosts up the gate voltage V_(G) of thedriving transistor M1 while making a transition from the scan period tothe emission period. It is assumed that a low level voltage of the scansignal is a low scan voltage VVSS, and a high level voltage of the scansignal is a high scan voltage VVDD. The voltage supplied to the firstelectrode of the auxiliary capacitor Caux is transitioned from the lowscan voltage VVSS to the high scan voltage VVDD, so that the gatevoltage of the driving transistor M1 is boosted up as much as acompensation voltage due to coupling between the storage capacitor Cstand the auxiliary capacitor Caux.

Finally, the gate voltage V_(G) of the driving transistor M1 can beobtained by Equation 2.Cst(V _(G) −Vg _(M1))=Caux{(Vg _(M1) −V _(G))−(VVSS−VVDD)}V _(G) =Vg _(M1) +Caux/(Cst+Caux)*(VVDD−VVSS)  Equation 2:where, V_(G) is a gate voltage of the driving transistor M1 afterforming the auxiliary capacitor Caux, Vg_(M1), is a gate voltage of thedriving transistor M1 before forming the auxiliary capacitor Caux, VVDDis the high level scan signal, VVSS is the low level scan signal, Cauxis the capacitance of the auxiliary capacitor, and Cst is thecapacitance of the storage capacitor.

Referring to Equation 2, after forming the auxiliary capacitor Caux, thegate voltage of the driving transistor M1 increases as much as thecompensation voltage of Caux/(Cst+Caux)*(VVDD−VVSS), therebycompensating the decreased voltage.

For example, when the first power supply voltage Vdd is equal to theblack data voltage, a very high black level current of about 7 nA flowsin the driving transistor M1 before forming the auxiliary capacitorCaux, so that a contrast ratio is greatly lowered. On the other hand,according to an embodiment of the present invention, a black levelcurrent of about 0.02 nA flows in the driving transistor M1 afterforming the auxiliary capacitor Caux, thereby satisfying thespecifications of the data driver, and enhancing the contrast ratio.Preferably, the capacitance of the storage capacitor Cst is larger thanthat of the auxiliary capacitor Caux. In the foregoing example, themeasured currents are obtained under the condition that the capacitanceof the storage capacitor Cst is larger than that of the auxiliarycapacitor Caux by ten times.

FIG. 5 is a circuit diagram of the detailed connection structure betweenthe representative demultiplexer of FIG. 3 and the representative pixelsof FIG. 4 according to an embodiment of the present invention, and FIG.6 is a timing diagram illustrating operations of the pixel circuit ofFIG. 5.

In FIG. 5, assuming that red (R), green (G) and blue (B) sub-pixels areconnected to one demultiplexer 151 connected to the 1^(st) output lineD1 (i.e., k=3).

Referring to FIGS. 5 and 6, a low level scan signal is supplied to the(n−1)^(th) scan line Sn−1 in the (n−1)^(th) scan period of onehorizontal period 1H. When the low level scan signal is supplied to the(n−1)^(th) scan line Sn−1, each initialization transistor M4 of the R, Gand B sub-pixels is turned-on. As the initialization transistor M4 isturned-on, a first terminal of the storage capacitor Cst and the gateelectrode of the driving transistor M1 are connected to theinitialization voltage line Vinit. That is, when the low level scansignal is supplied to the (n−1)^(th) scan line Sn−1, a previous framedata voltage stored in each storage capacitor Cst of the R, G and Bsub-pixels, i.e., the gate voltage of the driving transistor M1 isinitialized. Thus, when the low level scan signal is supplied to the(n−1)^(th) scan line Sn−1, the first switching transistor M2 connectedto the n^(th) scan line Sn maintains a turned-off state.

Then, a first switching element T1, a second switching element T2, and athird switching element T3 are turned-on in sequence by first throughthird control signals CS1, CS2 and CS3 sequentially supplied in the dataprogram period. First, when the first switching element T1 is turned-onby the first control signal CS1, the R data signal is supplied from thefirst output line D1 to the first data line D11. The first data linecapacitor C_(data11) then stores a voltage corresponding to the R datasignal supplied to the first data line D11. Next, when the secondswitching element T2 is turned-on by the second control signal CS2, theG data signal is supplied from the first output line D1 to the seconddata line D12. The second data line capacitor C_(data12) then stores avoltage corresponding to the G data signal supplied to the second dataline D12. Finally, when the third switching element T3 is turned-on bythe third control signal CS3, the B data signal is supplied from thefirst output line D1 to the third data line D13. The third data linecapacitor C_(data13) then stores a voltage corresponding to the B datasignal supplied to the third data line D13. The scan signal is notsupplied to the n^(th) scan line Sn during the data program period, sothat the R, G and B data signals are not supplied to the R, G and Bpixels.

Then, in the n^(th) scan period following the data program period, a lowlevel scan signal is supplied to the n^(th) scan line Sn. When the scansignal is supplied to the n^(th) scan line Sn, each first switchingtransistor M2 and each threshold voltage compensation transistor M3 ofthe R, G and B pixels are turned-on. Each first switching transistor M2of the R, G and B pixels respectively transmits the voltages Vdatacorresponding to the R, G and B data signals, which are stored in thefirst through third data line capacitors C_(data11) through C_(data13)during the data program period, to the R, G and B pixels. The thresholdvoltage compensation transistor M3 allows the driving transistor M1 tobe diode-connected. Through the diode-connected driving transistor M1, avoltage (Vdata−Vth_(M1)[V]) corresponding to the difference between thethreshold voltage Vth of the driving transistor M1 and the voltage Vdatacorresponding to the R, G and B data signals, which are stored in thefirst through third data line capacitors C_(data11) through C_(data13),is supplied to the gate electrode of the driving transistor M1 and thefirst terminal of the storage capacitor Cst. The voltage supplied to thegate terminal of the driving transistor M1 is based on Equation 1.

Then, when the n^(th) scan signal is changed to a high level and a lowlevel emission control signal is supplied to the emission control lineEn, the second switching transistor M5 and the emission controltransistor M6 are turned-on, so that the first power supply voltage Vddsupplied to the source electrode of the driving transistor M1 and adriving current corresponding to the gate voltage are supplied to theorganic light emitting diode through the emission control transistor M6,thereby emitting light with predetermined brightness. The gate voltageof the driving transistor M1 is based on Equation 2.

Thus, in the OLED according to an embodiment of the present invention,the R, G and B data signals sequentially supplied from one 1^(st) outputline D1 can be supplied to k data lines D11 through D1 k using thedemultiplexer 151. Furthermore, voltages corresponding to the datasignals are stored in the data line capacitors C_(data11) throughC_(data1k) in the data program period, and the voltages stored in thedata line capacitors C_(data11) through C_(data1k) are supplied to thepixels during the scan period. Thus, the voltages stored in the dataline capacitors C_(data11) through C_(data1k) are supplied to the pixelsat the same time, so that the data signals are supplied at the sametime, thereby displaying an image with uniform brightness.

Furthermore, the auxiliary capacitor Caux is provided in each pixel, sothat the lowering of the voltage supplied to the pixel due to chargesharing between the data line capacitor Cdata and the storage capacitorCst is avoided, thereby enhancing the contrast ratio.

In the OLED according to an embodiment of the present invention, red,green and blue organic materials are used to emit light. The red, greenand blue organic materials are different in emission brightnessaccording to the intensity of a current flowing therein. That is, thered, green and blue organic materials are different in efficiency, sothat the red, green and blue data voltage must be different so as toadjust white balance. However, when differences among the red, green andblue data voltages are large, it is difficult to drive the OLED.

Therefore, in the OLED according to an embodiment of the presentinvention, the red, green and blue sub-pixels have auxiliary capacitorswhich have different respective capacitances, thereby adjusting thewhite balance with the same data voltage.

FIG. 7 is a simulation graph showing changes in a driving currentaccording to the capacitance of the auxiliary capacitor in the pixel ofFIG. 4.

In FIG. 7, the horizontal axis indicates the capacitance of theauxiliary capacitor Caux, and the capacitance decreases as goes to theright. The unit of capacitance of the auxiliary capacitor Caux is PF(Picofarads). The vertical axis indicates a white current flowing in thepixel, and the unit of the white current is nA (Nanoamperes).

As shown in FIG. 7, the current in the pixel increases as thecapacitance of the auxiliary capacitor Caux decreases with respect tothe same data voltage. For example, when the auxiliary capacitor Cauxhas a capacitance of 0.05 PF, the pixel has a current of 10 nA. When theauxiliary capacitor Caux has a capacitance of 0.03 PF, the pixel has acurrent of 220 nA. When the auxiliary capacitor Caux has a capacitanceof 0.01 PF, the pixel has a current of 440 nA. Generally, the emissionefficiency of the organic material increases in order of green>red>blue.

Thus, when the red, green and blue sub-pixels have a white current ratioof about 2:1:4 to generate a white pixel, the auxiliary capacitors Cauxof the red, green and blue sub-pixels have a capacitance ratio of about3:5:1, thereby adjusting the white balance with the same data voltage.That is, the capacitance of each auxiliary capacitor is inverselyproportional to the white current ratio.

FIG. 8 is a layout diagram of red, green and blue sub-pixels capable ofadjusting white balance with the same voltage based on the simulation ofFIG. 7 according to an embodiment of the present invention.

Referring to FIG. 8, the red, green and blue pixels have the same layoutexcept the capacitance of the auxiliary capacitor. Furthermore, eachpixel circuit has the connection structure as shown in FIG. 4, and thelayout of FIG. 8 is as follows.

In the R pixel, the previous scan line Sn−1, the current scan line Sn,the emission control line En and the initialization voltage line Vinitextend parallel to one another in a first direction. Furthermore, thefirst power supply voltage line Vdd and the data line Dm extend parallelto each other in a second direction. The lines extending along the firstdirection and the lines extending along the second direction intersecteach other, with an insulating layer interposed therebetween.

As shown in FIG. 8, the first switching transistor M2, the drivingtransistor M1, the threshold voltage compensation transistor M3, thesecond switching transistor M5, and the emission control transistor M6are formed of a first semiconductor layer. Furthermore, theinitialization transistor M4 is formed of a second semiconductor layer.

The current scan line Sn functions as gate electrodes of the firstswitching transistor M2 and the threshold voltage compensationtransistor M3. The previous scan line Sn−1 functions as a gate electrodeof the initialization transistor M4. Furthermore, the emission controlline En functions as gate electrodes of the second switching transistorM5 and the emission control transistor M6.

The storage capacitor Cst includes an upper substrate and a lowersubstrate corresponding to the first power supply voltage line. Theauxiliary capacitor Caux includes a lower substrate corresponding to thecurrent scan line Sn and an upper substrate corresponding to the gateelectrode of the driving transistor M2. The auxiliary capacitor Cauxvaries in size according to the red, green and blue pixels. Thecapacitance of the auxiliary capacitor Caux can be adjusted by changingthe size of the upper substrate used as the gate electrode of thedriving transistor M2.

As shown in FIG. 7, when the red, green and blue sub-pixels have a whitecurrent ratio of 2:1:4, then the auxiliary capacitors Caux should have acapacitance ratio of 3:5:1, so that the white balance can be adjustedwith respect to the same data voltage. Therefore, as shown in FIG. 8,the auxiliary capacitors Caux(R), Caux(G) and Caux(B) of the red, greenand blue sub-pixels are different in size.

The capacitance of the auxiliary capacitor Caux varies according to theemission efficiency of each organic material. As the emission efficiencyof each organic material increases, i.e., the brightness increases withrespect to the same intensity of current, the capacitance of theauxiliary capacitor Caux should become larger. Generally, the greenorganic material has the highest emission efficiency, the red organicmaterial has the second emission efficiency, and the blue organicmaterial has the lowest emission efficiency. Therefore, the capacitanceof the auxiliary capacitor Caux is determined in order ofgreen>red>blue.

As described above, in the OLED according to an embodiment of thepresent invention, each pixel includes the auxiliary capacitor Caux, sothat the decreased data voltage supplied to the pixel by driving thedemultiplexer is compensated, and thus the contrast ratio is enhanced byallowing black to be displayed. Thus, there is no need to lower thepower supply voltage Vdd and Vss, and the DC/DC efficiency is notdecreased.

Furthermore, the capacitance of the auxiliary capacitor Caux variesaccording to each emission efficiency of the red, green and bluesub-pixels, so that the white balance is adjusted with the same datavoltage, and thus the OLED is easily driven.

Although the present invention has been described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that a variety of modifications and variations can bemade to the present invention without departing from the spirit or scopeof the present invention defined in the appended claims.

1. An Organic Light Emitting Display (OLED) having a plurality of pixelseach having a red sub-pixel, a green sub-pixel, and a blue sub-pixel,each sub-pixel comprising: a pixel driver connected to a data line, ascan line and a first power supply voltage line, and including a storagecapacitor adapted to store a driving voltage supplied via the data line,and generating a predetermined driving current; and an organic lightemitting diode connected between the pixel driver and a second powersupply voltage line, and adapted to emit light with a brightnesscorresponding to the driving current; and an auxiliary capacitorconnected between the storage capacitor and the scan line, and adaptedto generate a compensation voltage to increase the driving voltage;wherein the auxiliary capacitors of the sub-pixels have differentcapacitances according to an emission efficiency ratio of theirrespective sub-pixels.
 2. The OLED according to claim 1, wherein thecapacitance of the auxiliary capacitor is inversely proportion to adriving current ratio of the sub-pixels to generate a white pixel. 3.The OLED according to claim 2, wherein the auxiliary capacitor of thegreen sub-pixel has a larger capacitance than that of the auxiliarycapacitor of the red sub-pixel, and the auxiliary capacitor of the redsub-pixel has a larger capacitance than that of the auxiliary capacitorof the blue sub-pixel.
 4. The OLED according to claim 3, wherein acompensation voltage increased by the auxiliary capacitor is defined bythe following equation:Vx=Caux/(Cst+Caux)*(VVDD−VVSS) wherein, Vx is the compensation voltage,Caux is the capacitance of the auxiliary capacitor, Cst is thecapacitance of the storage capacitor, VVDD is a high level scan signal,and VVSS is a low level scan signal.
 5. The OLED according to claim 1,wherein the pixel driver further comprises: an initialization transistorconnected between a first terminal of the storage capacitor and aninitialization voltage line, and adapted to be turned-on by an(n−1)^(th) scan signal to initialize the storage capacitor; a firstswitching transistor connected to the data line, and adapted to beturned-on by an n^(th) scan signal to transmit the data voltage; adriving transistor having a first electrode connected to the firstswitching transistor and a gate electrode connected to a first terminalof the storage capacitor, and adapted to generate the driving current; athreshold voltage compensation transistor connected between the gateelectrode and a second electrode of the driving transistor, and adaptedto be turned-on by the n^(th) scan signal to cause the drivingtransistor be diode-connected and to compensate a threshold voltage ofthe driving transistor; and a second switching transistor connectedbetween the first power supply voltage line and the second electrode ofthe driving transistor, and adapted to be turned-on by an n^(th)emission control signal to transmit the first power supply voltage tothe second electrode of the driving transistor.
 6. The OLED according toclaim 5, wherein the pixel driver further comprises an emission controltransistor connected between the driving transistor and the organiclight emitting diode, and adapted to be turned-on by the n^(th) emissioncontrol signal to transmit the driving current to the organic lightemitting diode.
 7. The OLED according to claim 6, wherein the firstthrough sixth transistors comprise N or P MOSFETs having the sameconductivity type.
 8. An Organic Light Emitting Display (OLED),comprising: a display panel having a plurality of pixels adapted todisplay an image; a scan driver adapted to supply a scan signal to thedisplay panel; a data driver adapted to supply a data signal to thedisplay panel; a timing controller adapted to generate a data controlsignal to control the data signal and a scan control signal to controlthe scan driver; a demultiplexer coupled between the data driver and thedisplay panel, and adapted to sequentially supply the data signal tocorresponding sub-pixels that constitute the pixel; and a demultiplexercontroller adapted to control the demultiplexer.
 9. The OLED accordingto claim 8, wherein the demultiplexer is connected to the display panelvia data lines connected to respective sub-pixels.
 10. The OLEDaccording to claim 9, wherein the data lines are respectively connectedto auxiliary capacitors and wherein the auxiliary capacitors havedifferent capacitances according to their respective sub-pixels.
 11. TheOLED according to claim 10, wherein the auxiliary capacitors havedifferent capacitances corresponding to an emission efficiency of theirrespective sub-pixels.
 12. The OLED according to claim 10, wherein theauxiliary capacitors of the green, red, and blue sub-pixels haverespective capacitances in order of green>red>blue.
 13. The OLEDaccording to claim 8, wherein the demultiplexer sequentially suppliesthe data signals to each sub-pixel during a horizontal period.
 14. TheOLED according to claim 13, wherein the scan driver supplies the scansignal to the pixel, such that the data signals are simultaneouslyprovided to the sub-pixels.